Various new microfabrication techniques have been being developed to satisfy the recent requirements of increased integration and advanced performance of large-scale integrated circuits (hereinafter referred to as LSI) of semiconductor devices. Chemical mechanical polishing (hereinafter referred to as CMP) is one of them, and this is frequently utilized in LSI production, especially for insulating interlayer planarization, metal plug formation, buried wiring pattern formation and the like in multi-level interconnection, for LSI production. This technique is disclosed, for example, in U.S. Pat. No. 4,944,836.
Recently, copper alloys have been being tried for interconnection to realize high-performance LSI. However, copper alloys are difficult to use in a process of dry-etching microfabrication that is frequently used in interconnection with conventional aluminum alloys. For this reason, a damascene process is essentially employed for microfabrication with such copper alloys, which comprises depositing a thin film of a copper alloy on an insulating film having grooves previously formed therein, to thereby bury the copper alloy in the grooves, and thereafter removing the thin copper alloy film not buried in the grooves through CMP to form a buried wiring pattern of the copper alloy. This technique is disclosed, for example, in Japanese Patent Laid-Open No. 2-278822.
One general method of CMP of metal comprises sticking a polishing pad on a circular platen, soaking it in a metal-polishing liquid, setting a substrate having a metal film formed thereon to the polishing pad to keep the metal film in contact with the pad, and rotating the platen while a predetermined pressure (this is hereinafter referred to as a polishing pressure) is applied to its back to thereby remove the excess metal film owing to the mechanical friction between the polishing liquid and the hilled area of the metal film.
The metal-polishing liquid for CMP generally comprises an oxidizing agent and solid abrasive grains, optionally containing an oxidized-metal etchant and a protective film-forming agent. It is considered that the basic mechanism of CMP comprises oxidizing the surface of a metal film followed by scraping away the oxidized layer from the metal film by the action of solid abrasive grains. In the process of CMP, the oxidized layer of the metal surface in the grooved area of the metal film is not almost brought into contact with the polishing pad and is therefore almost free from the scraping action of the solid abrasive grains. Accordingly in this, the metal layer in the hilled area is removed through CMP, and the surface of the substrate is thereby planarized. The details of the process are disclosed in Journal of Electrochemical Society, Vol. 138, No. 11 (issued in 1991), pp. 3460-3464.
It is generally said that an oxidized-metal etchant, if added to the metal-polishing liquid, is effective for increasing the polishing rate in CMP. For the reason, it is understood that the oxidized-metal etchant added dissolves the metal oxide particles having been scraped off by the solid abrasive grains in the metal-polishing liquid to thereby enhance the scraping ability of the solid abrasive grains. The metal-oxide etchant added increases the polishing rate in CMP, while, on the other hand, it etches even the oxidized layer of the metal film surface in the grooved area, As a result, the metal film surface in that area is exposed, and it is then further oxidized with the oxidizing agent. After this is repeated, the metal film in the grooved area is thereby much etched. Accordingly, in a case where such an oxidized metal etchant is added to a metal-polishing liquid, the center part of the surface is depressed like a dish (this phenomenon is hereinafter referred to as dishing) and therefore could not be well planarized. To avoid this, a protective film-forming agent may be added to the metal-polishing liquid. In the metal-polishing liquid containing such a protective film-forming agent, it is important to well balance the effect of the oxidized-metal etchant with that of the protective film-forming agent in order that the etchant does not so much etch the oxidized layer of a metal film surface in the grooved area thereof but can efficiently dissolve the scraped oxidized layer particles so as to increase the polishing rate in CMP.
Adding such an oxidized-metal etchant and a protective film-forming agent to a metal-polishing liquid to expect their chemical reactions increases the CMP rate (that is, the polishing rate in CMP) and is effective for reducing the damage of the metal layer surface polished through CMP.
However, the following problems (1) to (4) are inevitable in the process of buried wiring pattern formation through CMP with a conventional metal-polishing liquid that contains solid abrasive grains.
(1) The center part of the surface of the buried metal wiring pattern is isotropically etched (dishing).
(2) The surface polished with the liquid is scratched by the solid abrasive grains.
(3) After polished, the surface must be washed to remove the solid abrasive grains still remaining thereon, but the washing operation is troublesome.
(4) The solid abrasive grains are expensive, and the waste treatment is costly. Accordingly, the cost of the CMP process itself is high.
In addition, most of the metal-polishing liquid is water. Therefore, the tank for transporting the liquid therein must be large, and the tank for storing it in a polishing plant must be also large. That is, the metal-polishing liquid requires a large space for producing, storing, transporting and using it, and this is a bar to automation in using the liquid in a polishing plant. Moreover, the cost of recycling the tanks used for transporting the liquid is extremely high, and this is still another problem.
The problems may be solved if a concentrate of the metal-polishing liquid not containing a large amount of solid abrasive grains can be prepared. The production costs in metal-polishing liquid makers could be reduced, and, as a result, the costs of the dilutions of the concentrate could also be reduced. In addition, mass-production of the concentrate does not require increasing the scale of the existing production plants, for which, therefore, no additional capital investment is needed. This is another advantage of the concentrate. In view of the advantages in using it, the concentrate is preferably prepared to have a degree of concentration of at least 10 times that of the diluted liquid thereof to be actually used in metal polishing.
On the other hand, for LSI interconnection of high reliability with neither dishing nor copper alloy etching in the polishing process, proposed is a method of using a metal-polishing liquid that comprises aminoacetic acid e.g., glycine or amidosulfuric acid serving as an oxidized-metal etchant, and benzotriazole (hereinafter referred to as BTA) serving as a protective film-forming agent. This technique is described, for example, in Japanese Patent Laid-Open No. 8-83780.
However, since the solubility in water of BTA is low (2 g in 100 cc of 20° C. water), some metal-polishing liquids of that type could not be concentrated into 10-fold concentrates (for example, the metal-polishing liquid containing 0.2% by weight of BTA can be concentrated into 5-fold concentrates, but if concentrated into 10-fold concentrates, BTA is deposited therein at 0° C. or lower). Accordingly, desired is a metal-polishing liquid containing BTA and capable of being concentrated into 10-fold or more concentrates, not forming a deposit of BTA in the concentrates even in ordinary environments at 0° C. or higher.